Body with a central disc of up to 8 mm in diameter and 5 arms of about 3 times that length; coloured reddish-brown or grey, with darker bands on the arms.
The disc is flat and regularly covered with fine scales. The primary plates on the dorsal side are round and very conspicuous. The radial shields are small and wholly separated. Along the side of the arms, two combs of papillae are visible. The outer arm combs consist of 7-9 short, flat, papillae, while the inner combs consist of 10 smaller ones (O. affinis disc).
The dorsal arm plates are very broad and roof-shaped. The ventral arm plates are widely separated, with an obtuse proximal angle and a very convex outer edge. There are 3 small arm spines per segment. The tube foot pores are partially covered by one broad tentacle scale. The first two proximal pores have a second tentacle scale on the outside (O. affinis arm).
The mouth shields are large and remarkably long. There is a single series of teeth on top of the jaw, and usually 3 mouth papillae on each side of the jaw. The outermost mouth papilla is very broad and scale-like (O. affinis mouth).
This species is found on muddy sand, fine shell and gravel bottoms, from about 8 to 550 m.
In the North Sea it is found from the Shetland Islands down to the Dogger Bank. Elsewhere it is distributed from Norway and the British west coasts down to the Bay of Biscay and the Mediterranean.